Because MgO displays excellent heat resistance, conventionally it has been used mainly as a heat resistant material for crucibles and fire bricks and the like, and various techniques have been proposed for improving the mechanical strength, including the addition of sintering assistants. Examples of known techniques include those disclosed in the patent references 1 to 3.
As follows is a list of references.    Patent Reference 1: Japanese Unexamined Patent Application, First Publication No. Hei 07-133149    Patent Reference 2: Japanese Patent No. 2,961,389    Patent Reference 3: Japanese Examined Patent Application, Second Publication No. Hei 07-102988    Patent Reference 4: Japanese Unexamined Patent Application, First Publication No. Hei 11-213875    Patent Reference 5: Japanese Unexamined Patent Application, First Publication No. Hei 05-311412    Patent Reference 6: Japanese Unexamined Patent Application, First Publication No. Hei 10-291854    Patent Reference 7: Japanese Unexamined Patent Application, First Publication No. Hei 10-297955    Patent Reference 8: Japanese Unexamined Patent Application, First Publication No. Hei 10-297956    Patent Reference 9: Japanese Unexamined Patent Application, First Publication No. Hei 11-29857    Patent Reference 10: Japanese Unexamined Patent Application, First Publication No. Hei 11-29355    Patent Reference 11: Japanese Unexamined Patent Application, First Publication No. 2000-63171    Patent Reference 12: Japanese Unexamined Patent Application, First Publication No. Hei 09-235667    Non-Patent Reference 1: IEICE Trans. Electron., vol. E82-C, No. 10, p. 1804–1807 (1999).
In recent years, the research, development, and implementation of flat panel displays, including liquid crystal displays (LCD) has been remarkable, and the production of these displays is increasing rapidly. The development and introduction of color plasma display panels (PDP) has also surged recently. PDPs can be easily produced in very large sizes, and represent the most promising technology for producing a high definition, wall-mounted large screen television. Tests and production of PDPs with a diagonal size in the 60 inch range are already in progress, and of the different PDP technologies, AC type display panels in which the electrode construction utilizes a metal electrode covered with a glass dielectric material are the most common.
In these AC type PDPs, the surface of the glass dielectric layer is coated with a protective film that displays a high heat of sublimation in order to prevent the ion bombardment sputtering from altering the surface of the glass dielectric layer and raising the breakdown voltage. Because it contacts the discharge gas directly, this protective film must not only be resistant to sputtering, but must also perform a plurality of other important roles. Namely, the properties required of this protective film include resistance to sputtering during discharge, a high secondary emission capability (a low discharge voltage), as well as good insulation and light transmittance. MgO films produced by either electron beam deposition or ion plating using MgO as the vapor deposition material are typically used as the material for satisfying these property requirements. As described above, these MgO protective films perform an important role in extending the life of the PDP by protecting the surface of the dielectric layer from the sputtering during discharge, and it is known that increasing the film density of this protective film enables an improvement in the sputtering resistance (non-patent reference 1).
In addition, both single crystal MgO and polycrystalline MgO have been used for this protective film (vapor deposition material). Technology relating to this protective film includes that disclosed in the patent references 6 to 11 listed above.
Furthermore, in a technique disclosed in the patent reference 4, the secondary emission efficiency of the protective film is improved, and display performance problems such as breakdown delay within the light emitting cells and faults within the write operation can be improved, by reducing the electronegative element component such as sulfur within the protective film.
The method used for removing the electronegative element component such as sulfur requires a heat treatment at a temperature of at least 1400° C. in either a vacuum with a pressure of no more than 1.33×10−1 Pa (10−3 Torr) or in an oxygen containing atmosphere, which is performed following crystal growth (pellet preparation) and prior to formation of the protective film, although in the case of polycrystals or single crystals produced using an electromelting method with a relative density of at least 90%, the efficiency of the sulfur removal was poor.
The polycrystalline MgO described above is typically produced by granulating, molding and baking an MgO powder of arbitrary purity and with an arbitrary impurities composition produced by either a sea water method or a vapor phase method. In contrast, single crystal MgO is typically produced by melting MgO clinker or light burned MgO (baked at a temperature of no more than 1000° C.) with a purity of at least 98% in an electric arc furnace, that is forming an ingot by electromelting, and then extracting single crystal portions from this ingot and crushing the product. The MgO powder, MgO clinker, or light burned MgO used as the raw material for producing these products typically comprises a sulfur S component, a chlorine Cl component, a nitrogen N component and a phosphorus P component and the like as impurities, due to the production methods employed and the intrinsic reactivity of MgO.
It is known that during film formation using either an electron beam deposition method or an ion plating method, the quantities of sulfur S, chlorine Cl, nitrogen N and phosphorus P discharged from the MgO vapor deposition material as impurities has a deleterious effect on the level of splash associated with that vapor deposition material. In other words, as the quantities of sulfur S, chlorine Cl, nitrogen N and phosphorus P impurities increase, the level of splash also increases, and if the level of splash is high, then the degree of efficiency with which the quantity of vapor deposition material can be used in the film formation process decreases, resulting in an increase in material costs.
In addition, if the quantities of sulfur S, chlorine Cl, nitrogen N and phosphorus P impurities are high, then control of the crystal orientation and microstructure of the product film becomes difficult, and compact deposition of the MgO film onto the substrate is inhibited, meaning the film density tends to decrease. If the density of the MgO film decreases, then various problems can arise including a reduction in the refractive index, a deterioration in the sputtering resistance, and deterioration in both the discharge characteristics and the insulating characteristics.
In terms of the target material used in the sputtering method, the type of technology disclosed in the patent reference 12 described above is already known.
When preparing a target material for use in a sputtering method, this target material must be produced with as high a density as possible. In contrast, the inventors of the present invention have confirmed that when preparing a vapor deposition material for use in an electron beam deposition method, the density required of the vapor deposition material need not be as precise in terms of the measurement of degassing from the vapor deposition material and the evaluation of the film formation, and the allowable density range for the vapor deposition material is comparatively broad.
Furthermore, the target material and the vapor deposition material are not significantly different in terms of their microstructures, but are very different in macro terms.
Namely, in a sputtering method, the target material is formed as a comparatively large plate such as a circular or angular plate, whereas in an electron beam deposition method, the vapor deposition material is formed as small pellets. In a sputtering method, in which positive ions such as argon cause knock-on at the atomic level of the target material, which functions as the film formation material, thereby depositing this film formation material on a substrate, the reason that the target material is formed as a plate that is comparatively larger than the substrate or the like on which the film is to be formed, is to ensure a more uniform distribution of the thickness of the film produced from the target material.
In contrast, in an electron beam deposition method, in which a material is deposited on a substrate by heating and subsequent conversion to a vapor state using an electron beam, the reason that the vapor deposition material is formed as small pellets is to enable the vapor deposition material to be supplied sequentially to the crucible of the electron beam deposition apparatus. Furthermore, in an electron beam deposition method, because of the so-called splash phenomenon peculiar to electron beam deposition and from the viewpoint of film formation speed, an optimum size exists for the vapor deposition process.
Because the film formation mechanisms for sputtering methods and electron beam deposition methods differ in this manner, the film formation conditions are completely different, and the quality of the product films is also different. Namely, in a sputtering method the composition of the product film varies depending on how easily the target material undergoes sputtering (the sputtering resistance), whereas in an electron beam deposition method the composition of the product film varies depending on how easily the vapor deposition material can be converted to a vapor state (the vapor pressure). As a result, the compositions of films formed from a target material and a vapor deposition material of exactly the same composition will differ, and the quantity of impurities incorporated within each film will also be different.
In addition, in the case of a target material, following formation of a plate-like material by sintering or the like, usually the surface must be polished using a surface grinder or the like to achieve a surface roughness of no more than approximately 1 μm. The reason for this requirement is that if sharp sections exist on the target material, then electrons tend to accumulate at those sections, meaning the argon ion bombardment is also concentrated on those sections, causing a phenomenon known as arcing, which causes non-uniformity in the product film, and consequently these sharp sections must be removed. In contrast in the case of a vapor deposition material, the film formation speed can be increased by ensuring as large a surface roughness as possible. The mechanism behind this finding is unclear, although it is thought that the increase in the evaporation surface area of the vapor deposition material is a significant factor.
Accordingly, a target material and a vapor deposition material only appear to be similar, and just because the compositions and/or forms of the two may be similar does not mean that either one may be appropriately applied within the other field.